If he has a hard (welded) short it will be bad - could be bad very bad
With lithium cobalt, that could be very bad with or without a BMS. A BMS can't cut-off a cell from itself; can't help a cell that shorts on the inside, or before its connection to the BMS.
But then you got PTC protection as all LI-ION cells have. Can't make the risk zero, not trying to. Just doing the customary and expected for a design of this type.
In the engineering world thats what is expected by the courts and of course you are still endlessly hassled (Been there done that discovery, testimony the whole shin bang) ). But if you want your insurance company on your side, then you research what is required by CE and UL and whoever else and do it right and document it (my weakness).
Now why do I care, the battery is just for me.
So I can sleep at night
Well if I get into a accident and that battery goes up in flames due to a impact. I'm in deep shit first Ebikes are illegal in NY wait I need to phrase this right, not allowed on the roads as per the DOT.
So its going to be a long legal battle to defend myself no matter who's fault it is. But with my engineering background in powersystems - not batteries though. I sleep well at night knowing I'm doing the best I can and also can show getting a battery cheap was not my direct goal but obtaining a light weight long range battery was my goal.
Saying my goal was to build something cheap is not going to cut it in the real world. It doesn't cost that much to add say 100 dollars to a design and stand up and say I did what the battery industry expects of a battery of this design. What would it take to put what you feel is the LV cutoff and put it into a circuit that either alerts you as a alarm system or a real cut off. Same with over current, its just not that hard and its the right thing to do.
Now for the most important reason, it was just plain fun, reverse engineering the C packs, taking them apart looking up the IC's then putting them back together and have them still work. Then seeing what I think is the weakness in my parallel pack design and designing and testing a system to deal with that weakness. Poor front brake, used it as a poor man Dyno for hours dropping out packs on over current restating them seeing the reverse charge effect 300mv delta in my packs creates about 3 to 5 amps of reverse current, so its diodes or a well designed protection system and of course fuses also test with enough reverse current so the fuses blow at 2C.
And the thrill of working on a live system
I can't say this enough
There is no off switch on a battery cell, you get a cell into thermal runaway and there is no fuel rods to push in no cooling water to turn on at the last second. Throw it in sand and hope for the best.
SO I PLAN ON DOING MY BEST WHEN MAKING A BATTERY
This isn't my last battery project, I'm hooked and maybe by next spring I might have a battery to sell completely of my own design